xtensa: Fix noreturn attribute on error handlers in asm2

In asm2, the machine exception handler runs in interrupt context (this
is good: it allows us to defer the test against exception type until
after we have done the stack switch and dispatched any true
interrupts), but that means that the user error handler needs to be
invoked and then return through the interrupt exit code.

So the __attribute__(__noreturn__) that it was being decorated with
was incorrect.  And actually fatal, as with gcc xtensa will crash
trying to return from a noreturn call.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>